I know DS understands basic things like "please sit on the couch" but what age do toddlers really start showing comprehension?

For example, DS wanted to be held while I cooked dinner which is fine. At one point I told him I had to set him down so I could pull something out of the oven. I would pick him back up as soon as our dinner was on the cooling rack. The moment I set him on a chair he started crying. The whole scenario was overdramatized because I don't think he understood what was going on.

I'd love to be able to sit next to him in bed and tell him to close his eyes and assure him that I'm not leaving. Or when he's super upset about x, I could ask him to listen to what I have to say.

I'm sure it varies, but at 15 months, I think both of my kids would have understood that. Maybe I'm getting old, but I'm having a harder time imagining DS at that age. DD was speaking in sentences by 13 months, so she definitely would've gotten it. DS didn't speak until later, but I'm still pretty sure he would have gotten it. Some of the issue may be the way you're phrasing things. Perhaps try simpler language. "One minute" while holding up one finger or something along those lines...

Also, it's possible that he did understand but just felt really clingy at that moment.

I think it take practice for them to understand what is happening. My dd is 2 and it took a while for her to understand things like 'be right back' or 'one minute'. I started by making these saying a regular part of her day, then it seemed to click after a week or so. I also make a point of giving her a distraction, or asking her where she wants to wait. It does happen, eventually!

My son (just days short of being 15months) would have probably understood the situation. However, around dinner time, he often is tired and hungry from the day; and I know, that in a similar situation he would have cried as well. But I also know when he's a happy toddler, it would have been no problem for him to be put down.

I think it is easier to learn new concepts like waiting when everyone is in a good mood and happy.

Mom to two sweet children - Oct'09 & May'12. Wife to a loving husband. Two little souls lost Oct'14(11weeks) and Mar'15 (13weeks). Expecting a little bean in Feb'16

My ds is 16 months. I think he really started listening and understanding between 8 months and a year.

That doesn't mean that he can always process and respond to the information in the way I would like.

As a pp said, waiting, especially at certain moments, just isn't going to happen. I don't think that means he doesn't understand what I'm saying to him. It is clear to me that he understands most of what I say to him.

He is capable of following instructions -- he just doesn't always do it.

I think dd was 2 or better before I could consistently rely on her to "wait a minute" when I asked her to. Heck, she's almost 4 and I still can't get her to stop talking to me when I'm on the phone. It's a continuum, and it gets better.